Hydrocarbon Deposits
The structures formed in sedimentary basins are known to act as traps for hydrocarbons (ie natural gas and petroleum). These deposits are highly sought after as they provide great wealth to those who exploit them. The processes that lead to the formation of these deposits are not well understood, but it is known that they are associated with rifting zones. As we have seen, rift zones correspond to regions under extension and therefore sedimentary basins. Hydrocarbon traps form in permeable layers of rock (such as sandstone) called reservoir rocks that are 'capped' by impermeable rocks (such as shale). Since water is denser than both petroleum and gas, they will not mix. Water will then force these fossil fuels through permeable rocks until they pool together in traps formed by cap rocks.
Illustrated below are just a few of the types of hydrocarbon traps that can be found in sedimentary basins. As we have seen already, the structural geology of an extensional basin can be quite complex. This is also true for the types of traps. Therefore the examples that will be given are simplified versions that are typical of those found in reality.
Fault traps
These traps are one of the most common to be found in extensional basins. They form when a normal fault causes the displacement of reservoir rocks such that where the bed would have originally continued on as a permeable layer, it is now cut-off by the fault and instead is capped by an impermable bed. Hydrocarbons such as natural gas and oil will pool in this trap, especially if it is dipping at some angle.
Pinch-out traps
These trap styles are also common in extensional basins. Unlike fault traps, these traps are purely the result of deposition processes rather than structural ones. They form as a reservoir rock layer simply tapers off into a cap rock. Water will then force hydrocarbons into these traps from below.
Anticlinal Traps
These traps are diagnostic of basins that have had some compressional forces applied on them that cause folding of the sedimentary layers. These traps are therefore not particularly common in basins that have only been formed in extensional regimes. However, as discussed in the previous section, when listric faults occur, roll-over anticlines may form. These structures if large enough may in fact become traps for hydrocarbons.
Other Trap Types
There are many other types of traps that are mined for hydrocarbons, too numerous to mention in this report. However, illustrated below are two other trap types common to extensional basins. In the centre of the image is an unconformity trap. These form when deposition of a cap rock occurs above tilted beds of reservoir rocks following a period of erosion. To the left of this is a tilted fault-block trap. These are typically formed by the rotation of domino faults mentioned in the previous section. If a permeable layer is now capped by an impermeable layer, an oil trap may form.